Well my temp rose during the summer at idle at a light, the heat outside was around 100F. Heat index near 110F. I usually keep an eye on that to make sure that it doesn't get into the RED ZONE. Once I got moving back out on the road the temperature went back down. Now at this time with the weather cooler my temp hasn't gotten past 85F. I had an indy make sure that the electric fan was working, he put a test on it and it came on. Now it may not be coming on at all when i'm setting at idle, i'm not sure. I've not seen any sign of coolant loss at all. It's remained normal and no signs of oil in the coolant tank. If I had my guess if my head was cracked i'd know it.

Tom,

Even in high ambient temps, the car shouldn't idle above 100-105C. I wouldn't have believed it myself if my car hadn't done that after getting the new radiator... and that was when I lived in Sacramento, where summer temps can reach 110F+. Mine would run plenty cool all winter, but ran *way* too hot in summer (105-115C!), the new rad+clutch was the cure (90-95C all the time, summer included.)

About the electric fan. It's easy to test the fan - that's probably what your indy did. And it's fairly easy to test if it's working at low speed, just turn the AC on when it's hot out, and within 5-10 minutes the fan should turn on low speed. If not, the refrigerant pressure switch is bad - replace the old green switch at the drier with the new-style red one, which turns it on at a lower pressure, and improves cooling. (You have to discharge the system to do this - d'oh.)

It's much harder to test the high-speed fan switch. This is triggered by the 3-prong switch at the head coolant exit, where the upper radiator hose connects. It should turn the fan on high speed when coolant temp reaches 105C. My car never turned that fan on - a new switch cured it (this was before I fixed the radiator, of course). Pretty much every other 603 I have seen has this switch not working, if it's original. It's really not worth troubleshooting, if you know it's not recent, just spend the $25 for a new one and sleep peacefully.

Finally - in the early stages of the head crack, there is NO coolant loss, and NO fluids mixing. Just high pressure in the system when cold. If/when the crack gets worse, the other symptoms may appear.

__________________
Dave
Boise, ID
Check out my website photos, documents, and movies!

Since we are talking about temperatures again, I thought I would post the interesting results of the SDL in cooler weather.

When warmer, the temperature gauge would function like a boost gauge. If I used boost for 30 seconds or more, the gauge would climb right up from 90 or so and head toward 100. Would never go above 100, with the one exception of a five mile upgrade in West Virginia when it made it to 105 under full boost and 80 degree ambient.

So, now, when colder, it behaves in a very similar fashion, only a bit cooler. It typically sits at 87 or so, under limited or no boost. Under heavy boost, it does its usual climb up to about 95-96 or so. Remove the boost and it returns to its typical 87-89 setting in the colder weather.

I'm now convinced that this behavior is normal for this engine and there is a differential between the thermostat and the guage. The function of the thermostat must allow slightly higher temperatures under heavy loads and that is what the guage is showing. I conclude this because the cooling system has ample capacity to keep the engine precisely at 90 degrees if the thermostat would allow it.

The only other possibility is that the thermostat is defective and is not able to precisely control the outlet temperature anymore.

Thoughts?

Brian,

Your car operates almost *exactly* like mine does, and yes I believe this is absolutely normal for a 603 turbo. Note that neither of ours exceeds 105C even under heavy loads in summertime. That's why I say if it's getting over 105C, under load OR at idle, something ain't right...

Dave I should change out the Refrigerant Pressure Switch and the High Speed Fan Switch? Temp Sensor as well?

I'm pleased to report that both of my 300D's have passed all the head tests that I currently know of. Yesterday I did the idle test with the pressure tester and there was no vibration of the pressure needle, even at 2-3K rpm. Then later in the evening I took both cars on a 20 min run including two full-throttle freeway onramp runs. Thanks GSXR for letting me know about this test. This morning the hoses were soft.

I have to emphasize that for the cold hard hose test to be valid you really need to make sure there are no leaks in the cooling system. I did that with my pressure tester, but you also need to make sure the cooling system cap is seated properly. On my W124 I found out that if I just slap it on it will leak air. I found out by putting my ear on the cap and squeezing the upper radiator hose. I had to partially open it and then close it again to seal it. I think it's the cap seat's fault because the cap is only a year old.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

It is very common for seals to not seal very well without a pressure differential.

I would check the seals on radiator cap very closely for small cracks/tears and debris, especially the pressure relief valve gasket. I have found several bad caps, that were causing me problems, one had debris on the seal that was hard to see, and one was cracked, neither would draw coolant out of the resevoir. The latest, was on my 300D, and I think it may have masked the early cold pressure symptoms.

Tom, if the high-speed fan switch (the three-prong, 105/120C switch) is old or original, yes I'd replace it. Part number is 006-545-61-24, about $25 or so, this is the recommended switch for all 1986-87 OM603 turbos.

If you have a "green" refrigerant pressure switch that is working OK, it should be replaced with the new "red" switch the next time the AC system is evacuated for service. Otherwise, it's probably not worth the hassle to discharge & recharge the system just to change the switch - unless it's bad, of course. The newer red switch will trigger about 60psi lower, making the fan run more often, improving AC performance a little bit at idle speeds. The switch is cheap but AC service isn't...

From memory, the green switch turns the fan on at 20 bar and off at 16 bar. The red switch turns the fan on at 16 bar and off at 12 bar. HUGE difference there. And not to sidetrack this thread, but if your car still has R-12, pay whatever it takes to KEEP it R-12. The R-134a conversion is the kiss of death to any original R-12 system, and I know more than one person who converted back (at great expense) due to the inferior cooling with the R-134a.

DA, that's good news! IF you have no coolant loss and a soft hose in the morning, that's perfect. You are 100% correct that the soft hose is ONLY meaningful if the system will hold "normal" pressure (20-25psi) and loses NO coolant.

It might be here someplace.. But where does one see what the head number is for their head??? Where is it stamped???

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1991 350SDL
1978 300D
1995 Chev Conversion Van
2001 Mazda PU
1966 Chrysler New Yorker
1994 Lincoln Town Car

It's normally part of the casting on the side of the head that faces the manifold. It is beneath the #2 runner on the manifold and you will need an angled mirror to read it.

There are a bunch of numbers there. You are interested in the second to last group of two. The earliest is "14". The latest is "22".

I bought a SDL with 166,000 miles and a -14 head. After 4000 miles it started to eat coolant and exhaust white smoke. It never overheated while I owned it. I decided to spend the $3000+ to replace the head with a new one (-22). When I pulled the old head it had cracks on three cylinders, between the valves and between the valves and the prechambers. I suspect the previous owner overheated it at some time, even though I was told it was never overheated. In hindsight, I should of suspected something because the radiator, water pump, and thermostat were all replaced prior to the sale.

It runs well now, but I have invested considerable more money in it than I had originally planned. I wish I had been more familiar with this forum before I bought the car. I probably paid $3000 too much. However, I now have a great running SDL and am looking forward to many years of service.

This is rather interesting. The PO replaced the radiator, water pump, and thermostat prior to the sale. Is it possible that he overheated it just before selling it? Are you positive that the engine was OK for 4,000 miles before any symptoms began to show up?

This might shed some light on how long it takes for cracking to develop after a serious overheat event.

Mack,

Uh-oh - that's not good. Is the car still parked? Are you going to swap heads on it? BTW, it's very wise to stop driving the car when it begins to "burn" coolant. I have heard of more than one incident where the owner kept driving it, and when the head was finally removed, the block surface was steam damaged (pitted, corroded) from the leak, and required a new short block!! (Usually a used engine is installed, but you get the idea - the block was toast.) While this is not terribly common, it's something to keep in mind - when coolant is vanishing with NO signs of external leakage on a 603, it's time to park the car and yank the head.

__________________
Dave
Boise, ID
Check out my website photos, documents, and movies!

Thanks Dave, I'll change that out probably after Christmas. I need to wait till then, i'm low on cash